The present invention concerns a method for modifying, combining and controlling the exhausts of a multi-cylinder internal combustion engine while operating to take advantage of the dynamic pulse effect of the exhaust flows to give an increase in volumetric efficiency, power and performance over a wide range of engine speeds.
Previous methods with multi-cylinder internal combustion engines take advantage of the exhaust gas pulses to improve the volumetric efficiency by having the resonance frequency of exhaust gas pulses at an optimum value for the extraction of the exhaust gases from the engine at the desired operating engine speed. Consequently at that engine speed, the resonance of exhaust gas pulses in the exhaust pipe will enhance the extraction of the exhaust gases from the engine and so improve the volumetric efficiency and power output. However, since the optimum resonance frequency varies for different engine speeds, the previous systems are only effective at a certain engine speed and at other speeds do not enhance the extraction efficiency and may even cause a back-pressure pulse that lowers the extraction efficiency below that which would be obtained if an exhaust pipe were not used.
For example, an exhaust system designed for high speeds does not provide desired dynamic effect at medium speeds. When the engine speed (Ne) drops to a medium speed, the volumetric efficiency of the engine is reduced because the dynamic effect does not match the exhaust timing. As a result, the output of the engines (Ps) will be reduced at medium speeds as shown by the continuous line in FIG. 1.
This problem has been solved to some extent with a new system described in the Japanese Utility Model Application Laid open with No. 55-25602 of 1980. This system features selector valves for combining exhaust flows according to the engine speed. That is by either integrating all the exhaust gas flows at the outlet or joining four pipes to two pipes and then two pipes to one pipe in the case of a four cylinder engine (4-2-1 exhaust passage combination), as a method to minimize the power output reduction due to the back-pressure pulse at the out of range engine speeds.
Another system has also been proposed in Japanese Patent Application Laid Open No. 60-128921 to protect against power reduction at the out of range engine speeds by varying the exhaust characteristics by changing the effective cross-sectional area of the exhaust pipes with the change in engine speed.
This applicant also filed Japanese Patent Application No. 62-61628 in 1987 to protect against the exhaust pressure of a 4 cycle, 4 cylinder internal combustion engine from being increased by the sudden change in passage area, by forming a so-called 4-2-1 exhaust passage combination resulting in an increase power characteristics, and by changing the combinations of the exhaust pipes to combine either pipes from cylinders at a 180.degree. ignition phase difference or pipes from cylinders at a 360.degree. ignition phase difference depending on the rotation speed of the engine.
Another system described in Japanese Patent Application Laid Open No. 62-12824 of 1987 features control valves located between exhaust pipes and the exhaust expansion chamber. These valves control the exhaust flows according to the engine speed to minimize power output reduction at out of range speeds. The system can be designed to offset undesirable back-pressure pulses at medium speeds as shown by the dotted line in FIG. 1.
However, all these methods are only effective in reducing the negative effect of the exhaust pulses at engine speeds outside the optimum speed for the system and are unable to improve the performance of the engine over a wide range of engine speeds by taking advantage of the exhaust pulse effect.
Also, the latter system is not applicable to motorcycle applications for the following reasons:
Normally, exhaust pipes are located under the frame and engine. If the above control valves are installed on these exhaust pipes, they will be exposed above the ground and may be damaged by rebounding rocks and other objects.
A standard expansion chamber is located on the rear side of the engine. The control valves are located between the expansion chamber and the exhaust pipes laid along the engine. When the motorbike is traveling, air may enter the exhaust pipes. Due to the relative positions described above, this air is heated by the engine and exhaust pipes before it reaches the valves. As a result, the valves cannot be cooled enough for normal operation. To solve these problems, additional improvement is required.
The method of modifying, combining and controlling the exhausts of multi-cylinder internal combustion engines of the present invention is designed to solve all of these problems.